Treatment of lignocellulosic material with organomercaptan



United States Patent 3,490,992 TREATMENT OF LIGNOCELLULOSIC MATERIAL WITH ORGANOMERCAPTAN Carl A. Johnson, Toledo, Ohio, assignor to Owens-Illinois, Inc. a corporation of Oh'o N0 Drawing. Filed Dec. 30, 1966, Ser. No. 606,012 Int. Cl. D21c 3/04, 3/02, 3/20 U.S. Cl. 16276 13 Claims ABSTRACT OF THE DISCLOSURE Methods of pulping lignocellulosic material such as hardwood and softwood by a two-stage treatment which includes the steps of (A) digesting the lignocellulosic material with a treating liquor containing an organomercaptan such as thioglycolic acid, the treating liquor having an initial pH within the range of 7.6 to about 12.0, the temperature and time of digestion being sufficient to convert the lignocellulosic material to a treated material containing mercaptan-reacted lignin; and (B) extracting the mercaptan-reacted lignin retained by the digested material by contacting it with a dilute solution of a watersoluble inorganic base such as sodium hydroxide.

This invention relates broadly to the art of treating lignocellulosic material including both softwoods and hardwoods. More particularly it is concerned with a treatment leading to the pulping of such material and which involves two main stages or steps: (a) a digestion or cooking step and (b) an extraction step. (For convenience and brevity such a treatment is sometimes referred to hereafter and in the appended claims as a two-stage treatment.

Still more particularly the invention is concerned with the treatment of lignocellulosic materials by first digesting such material with a treating liquor containing an organic thio compound, especially an organomercaptan, e.g., thioglycolic acid (TGA), HSCH COOH, thereby to obtain a digested or cooked material containing mercaptan-reacted lignin; and subsequently extracting mercaptan-reacted lignin retained by the digested lignocellnlosic material by contacting it with a dilute solution of a water-soluble inorganic base, e.g., a dilute aqueous solution of sodium hydroxide. The treating liquor used in the digestion step is characterized by having an initial pH within the range of from 7.6 to about 12.0.

It was known prior to the present invention to digest or cook WOOCl, specifically spruce sawdust, with an organomercaptan, more particularly TGA. See, for example, Ingeniors Vetenskaps Akademien, Proceedings No. 103, 77 pp. (1930), The Mercaptans of Pinewood, by Bror Holmberg. Holmbergs procedure was to treat, for instance, spruce sawdust with a solution containing TGA and which solution had been .made strongly acidic with hydrochloric acid. In a second step the mercaptan-reacted lignin was extracted by treating the digested wood with an aqueous solution of caustic soda.

Currently lignocellulosic material, specifically pine wood, is pulped (i.e., digested or cooked) by the kraft process (yield, about 52%) to obtain paper and other finished cellnlosic products having optimum tensile and tear properties. If cooking conditions are changed to obtain higher yields of pulp, the properties of the resulting pulp are poor. Surprisingly and unobviously it was found that, by practicing the present invention as broadly described in the first two paragraphs of this specification and more specifically hereafter, pulps having good properties can be obtained in a high yield. For example, it has been found that pine pulps with good pulp characteristics can be obtained at pulp yields of about 70%. It was also found that the same process could be used for pulping a hardwood, e.g., aspen and black gum woods, as was employed in pulping a softwood. In each case the yields of pulp were high, and papers made therefrom had good tensile properties.

The foregoing discoveries are of considerable economic and practical importance and significance. For example, the adaptability of the process for pulping both hardwoods and softwoods makes it possible to locate a single plant of the same design (instead of two or more plants of diiferent designs) in the same forest land area where both softwoods and hardwoods are available for cutting and processing. The advantages of being able to obtain pulps having good paper-making properties in higher yields will be apparent to those skilled in the art.

In practicing the present invention any wood or other lignocellulosic material, or mixtures thereof in any proportions, may be cooked or digested with a treating liquor containing an organomercaptan and having a controlled alkaline initial pH. The treating liquor is brought into intimate contact with the lignocellulosic material either with or without first removing the extractives by treating the lignocellulosic material is sub-divided form (e.g., in the form of sawdust, shavings, wafers, and/ or chips) with an organic solvent capable of extracting the organic solvent-soluble components of the material. Such lignocellulosic materials include softwoods, hardwoods and fibrous annual plants.

Examples of softwoods are balsam fir, eastern hemlock, jack pine, eastern white pine, red pine, black spruce, red spruce, white spruce, tamarack and cypress. Examples of hardwoods are black gum, quaking aspen, mixed tomahawk, American beech, paper birch, yellow birch, eastern cottonwood, sugar maple, silver'maple, yellow poplar, black cherry and white oak. Examples of fibrous annual plants are bagasse, hemp and jute. Mixtures of woods or other lignocellulosic materials of different origin may be used if desired, e.g., mixtures of diiferent softwoods, or of different hardwoods, or of one or more softwoods and one or more hardwoods.

THE ORGANOMERCAPTAN \REACTANT Illustrative examples of organomercaptans that may be employed in digesting wood or other lignocellulosic material in practicing this invention are those embraced by the general formula I wherein R represents a divalent radical, more particularly a divalent hydrocarbon radical, Y represents a monovalent substituent bonded directly to R, and n represents a numeral ranging from 0 up to the combining power (i.e., a value that will completely satisfy all valences) of the divalent radical represented by R.

Illustrative examples of divalent radicals represented by R in Formula I are divalent hydrocarbon radicals and, more particularly, divalent aliphatic, especially divalent saturated aliphatic, e.g., ethylene, propylene (trimethylene), butylene, isobutylene, pentylene, isopentylene, decylene, etc., including divalent cycloaliphatic, especially divalent saturated cycloaliphatic, e.g., cyclopentylene, cyclohexylene, cycloheptylene, etc.; divalent aromatic, e.g., phenylene, naphthylene, etc.; divalent adiphatic-substituted aromatic, e.g., 2,4-tolylene, ethyl- 2,5-phenylene, isopropyl 3,4 phenylene, 1-butyl-2,4 naphthylene, etc.; divalent aromatic-substituted aliphatic, e.g., .phenylethylene, phenylpropylene, naphthylisobutylene, xylylene, etc.; and radicals that may be classed either as divalent aromatic-substituted aliphatic or divalent aliphatic-substituted aromatic, e.g., 4,alpha-tolylene,

!,beta-phenyleneethyl, 4,alphaxylylene, 2,gamma-phenyl- :nebutyl, etc. Thus R may represent a divalent hydro- :arbon radical represented by the general formula.

wherein Ar represents an arylene radical and R represents an alkylene radical. Preferably the divalent hydro- :arbon radical represented by R contains not more than 10 carbon atoms, more particularly from 1 to 8 carbon atoms.

Preferably, also, the divalent radical represented by R in Formula I is free from olefinic or acetylenic unsaturation either in a straight chain or in a side chain.

It is not essential that the divalent radical represented by R be composed solely of carbon and hydrogen atoms. For example, the chain of carbon atoms, whether straightchain aliphatic or carbocyclic, may be interrupted in the chain by other atoms, e.g., by oxygen and/or sulfur and/ or nitrogen atoms bonded directly to carbon atoms of the chain.

Illustrative examples of substituents represented by Y in Formula I are functional groups such as OH; CN; -SH; COOH; COOR, wherein R is a monovalent hydrocarbon radical corresponding to the divalent hydrocarbon radicals represented by R in Formula II; COOM, wherein M is a salt-forming cation, e.g., --NH or Na, K, Li or other alkali metal, a saltforming amine such as a mono-, di-, or tri-(hydrocarbonsubstituted) or (hydroxyhydrocarbon substituted) amine, or other salt-forming cation and especially those which yield water-soluble salts when present in the particular thio compound. Or, Y may be a radical represented by wherein R" and R' are members of the group consisting of hydrogen and monovalent hydrocarbon radicals corresponding to the divalent hydrocarbon radicals represented by R in Formula I.

It will be understood, of course, by those skilled in the art that when n in Formula I represents zero then there are no radicals represented by Y in the formula, which latter then becomes (III) HSR wherein R represents a monovalent radical, more particularly a monovalent hydrocarbon radical corresponding to the divalent hydrocarbon radicals represented by R in Formula I. Illustrative examples of mercapto compounds embraced by Formula III are the alkyl (including cycloalkyl), aralkyl, aryl and alkaryl mercaptans, more particularly those which contain from 1 through carbon atoms and especially those having not more than about 8 carbon atoms.

The relatively low water-solubility of the unsubstituted hydrocarbyl mercaptans embraced by Formula III makes them much less suitable for use than substituted hydrocarbyl mercaptans having one or more polar or solvating substituent groups. However, if water-solubility of the mercapto pulping agent is unimportant, e.g., when it is to be used is undiluted state, or in solution in an organic solvent (e.g., ethanol) or in a mixture of water and an organic solvent in which mixture the unsubstituted hydrocarbyl mercaptan is adequately soluble, then mercaptans within the scope of Formula III may be employed as the pulping agent.

Particularly useful in practicing the present invention are organomercaptans represented by the general formula (IV) I-ISZ(COOR) wherein Z represents an alkylene (including cycloalkylene) radical containing from 1 through 10, and preferably from 1 through about 8, carbon atoms; R represents a member of the group consisting of (a) hydrogen, (b) alkyl radicals containing not more than about 10 carbon atoms and preferably a lower alkyl radical (e.g., an alkyl radical containing from 1 through about 6 carbon atoms); and (c) a salt-forming cation, examples of which have been given hereinbefore with reference to M in the grouping COOM which may be a substituent represented by Y in Formula I; and n represents an integer from 1 up to that of the combining power of the alkylene radical represented by Z. The alkylene radical represented by Z may be straight-chain, branched-chain, or cyclic as in, for example, cyclopentyl, cyclohexyl and the like.

More specific examples of mercapto compounds embraced by Formula IV are monocarboxylic and polycarboxylic acids such as those having the formulas the ammonium, alkali-metal (sodium, potassium, lithium, etc.) and other water-soluble salts of the aforementioned monoand dicarboxylic acids; and the cyclopentyl and cyclohexyl esters, as well as the methyl, ethyl and propyl I through pentyl (normal or isomeric alkyl) esters of the aforesaid acids. In the case of the salts and esters of the dicarboxylic acids, one may use either the monoor di salt, or a mixture thereof, or a monoor diester, or a mixture thereof.

As the organomercaptan digestion or pulping agent it is preferred to use thio acids, or salts or esters thereof represented by the general formula I wherein n represents an integer from 1 to 6, inclusive,

more particularly from 1 to 4, inclusive, and R has the same meaning as given above with reference to Formula IV. Thus, compounds embraced by Formula XI 'may be the thio acid itself or a salt (especially a water-soluble salt) or an ester of such an acid. Of these compounds, thioglycolic acid and the water-soluble salts and the lower alkyl esters thereof are the more preferred sub-group. Mixtures of acids and/ or salts and/ or esters embraced by Formula XI may be used if desired.

Instead of using organomercaptan compounds that are within the scope of Formula XI, one may employ those wherein the COOR group in that formula has been replaced by other hydrolyzable or solvating groups such as OH, CN,

RI/I and o R t wherein R" and R' in the last two groups are hydrogen or a monovalent hydrocarbon radical corresponding to one of the divalent hydrocarbon radicals represented by R in Formula I.

THE SOLUBLE INORGANIC BASE Any soluble, more particularly water-soluble, inorganic base (including salts and other substances that are hydrolyzable to an inorganic base) may be employed (singly or a plurality thereof) as an extractant of the mercaptanreacted lignin from the lignocellulosic material which has been digested with a treating agent comprising an organomercaptan. By inorganic base is meant any inorganic compound or substance that, at some concentration, will impart alkalinity (i.e., a pH above 7.0) to a solution thereof in water at 25 C. By water-soluble base is meant any inorganic base that has at least some solubility in water.

Illustrative examples of suitable inorganic bases include the alkali-metal (especially sodium and potassium, but preferably sodium for economical reasons) hydroxides and carbonates (including the bicarbonates); the alkalimetal phosphates, silicates and borates, e.g., sodium metasilicate, trisodium phosphate and the various sodium tetraborates including borax; alkali-metal salts of organic acids, e.g., sodium and potassium acetates; oxides, hydroxides and soluble carbonates of the alkaline-earth (including magnesium) metals, e.g., saturated aqueous solutions of CaO and MgO and/or the hydroxides of these alkaline-earth metals; and others that will be apparent to those skilled in the art from these illustrative examples.

Preferably the inorganic base is an alkali-metal hydroxide or carbonate which is used in the form of a dilute aqueous solution. The concentration of the base in the solution is important, especially when a strong base such as sodium hydroxide is employed. In such a case and in order to minimize hydrolysis of the cellulose in the digested lignocellulosic material, it is preferred to use a dilute aqueous solution of sodium hydroxide that contains from 0.5 to 10, more particularly from 0.8 to 2, weight percent of NaOH based on the weight of the oven-dried (O.D.) ligno-cellulosic material.

The aims, objects and purposes of this invention, including those indicated in the third and fourth paragraphs of this specification, are attained by providing a method of modifying or altering (more particularly, pulping) lignocellulosic material by a two-stage treatment which includes the combination of two essential steps A and B.

In step A the lignocellulosic material is cooked or digested with a treating liquor containing an agent reactive with the lignocellulosic material. This agent is comprised of an. organomercaptan in an amount corresponding to at least 5, preferably at least about 10 (e.g., from 10 to or more), weight percent based on the weight of the OD. lignocellulosic material. The best results are obtained using about 20 weight percent (on this same basis), although beneficial effects are secured when the organomercaptan is employed in amounts substantially above 30 weight percent, e.g., 50 or even 100 weight percent or more based on the weight of the OD. lignocellulosic material. Economic considerations, especially the cost of the organomercaptan and the amount thereof recoverable from the process, largely determine the particular amount of organomercaptan that is obtained for a given result from its use in that amount. Otherwise stated, no more organomercaptan with respect to the amount of 0D. lignocellulosic material should be used than is necessary to obtain the optimum yield of the desired product with optimum properties at minimum cost of time and labor.

The treating liquor has an initial pH Within the range of from 7.6 to about 12.0. The minimum pH of 7.6 assures that the liquor will be definitely on the alkaline side when cooking or digestion is started. The maximum initial pH of about 12.0 is such as will provide for the desired two-stage treatment and thereby minimize hydrolytic attack of the alkaline treating liquor upon the cellulose molecule in the lignocellulose during the digestion treatment. Too high a pH also must be avoided in order to minimize degradation of the organomercaptan, e.g., TGA, by the alkali in the treating liquor, especially at the relatively high treating temperatures that are normally employed. The temperature and time of digestion of the lignocellulosic material in the treating liquor are sufficient to convert the said material to a treated material containing mercaptan-reacted lignin.

In step B of the two-stage process with which this invention is concerned, the mercaptan-reacted lignin retained by the digested lignocellulosic material is extracted by contacting it with a dilute solution of a water-soluble inorganic base, numerous examples of which have been given hereinbefore.

For economical reasons and to simplify the recovery of organomercaptan for re-use in the process, the method generally includes the step of removing the excess treating liquor from the treated material from step A, after which the residue that remains is preferably washed with a washing fluid, e.g., hot water, before the extraction step described as step B is carried out.

THE DIGESTION STEP The treating liquor used in practicing this invention contains a reactive agent comprised of at least one organomercaptan which is dissolved, at least under the reaction conditions of temperature, pressure, pH, etc., in an aqueous liquid reaction medium, specifically water. The organomercaptan is preferably thioglycolic acid (TGA) and/ or a water-soluble salt thereof.

The concentration of the organomercaptan in the aforesaid liquid reaction medium cannot be stated with precision since it is dependent upon so many different influencing factors including, for example, the kind of wood or other lignocellulosic material being digested; the nature of its subdivided form; the ratio between the organomercaptan and the lignocellulose; the temperature and pressure of digestion; type of digester employed; and other influencing factors. Generally, however, the organomercaptan constitutes, by weight, from about 1% to about 14%, more particularly from about 2% to about 6%, of the weight of the liquid reaction medium.

The digestion of the lignocellulosic material in the treating liquor is at least initiated under alkaline conditions. When the digestion is started, the treating liquor therefore should be distinctly basic (as distinguished from neutral or approximately neutral); that is to say, the liquor should have an initial pH of at least 7.6. However, the initial alkalinity or basicity of the treating liquor should not exceed a maximum pH of about 12.0 for the reasons previously mentioned. Preferably the treating liquor has an initial pH within the range of from about 8 to about 11.

i For alkalinity-control (specifically pH-control) of the initial treating liquor one may employ an alkali-metal hydroxide, preferably sodium hydroxide, or other suitable inorganic base. Numerous examples of such bases have been given hereinbefore with reference to the inorganic-base component of the liquid extractant used in the extraction step. Alkaline organic compounds also may be used, if desired, for providing the desired initial pH of the treating liquor, e.g., the various amines, quaternary ammonium compounds and other nitrogenous bases, and preferably those having a boiling point or boiling range above the maximum digestion temperature under the particular pressure conditions prevailing during the digestion period.

The cooking or digestion of the lignocellulosic material with a treating liquor comprising an organomercaptan may be effected at a temperature within the range of, for example, from a minimum of 50 C. (more particularly 70- C. and preferably a minimum of at least about C.) to 200 C., more particularly from about to 190 C., and still more particularly at from about -180 C. The time of treatment varies, for instance, from /2 to usually 4 or 5 hours but which sometimes may be 6 or 8 hours or more at the maximum treating temperature. This time period and the reaction temperature depend upon such influencing factors as, for instance, the

type and degree of sub-division of the lignocellulosic material being treated, the chosen organomercaptan, the amount of the organomercaptan with respect to the lignocellulosic material, the concentration of the organomercaptan in the treating liquor, the type and size of digester used, the type of product desired, and other influencing factors.

The liquor recovered from the initial digestion with the treating liquor containing an organomercaptan may contain a significant amount of unused cooking chemicals. This liquor may be recycled in the process after making up for the spent chemicals; or the residual chemicals may be separated from the liquor by such methods as ion exchange, solvent extraction, distillation, dialysis, etc., for re-use in the process.

OTHER STEPS IN THE METHOD In the two-stage method of this invention the excess liquor is preferably removed (e.g., by draining) from the reactor or digester at the end of the cooking period, and the residue is washed, e.g., with water and, preferably, hot water.

The washed, subdivided, residual wood containing organomercaptan-reacted lignin is then either transferred to an extraction vessel; or the digester in which the initial cooking was carried out may be used as the extraction vessel. Q

Organomercaptan-reacted lignin retained by the residue (preferably the residue remaining after draining off the excess liquor; and, more preferably, the residue that remains after washing the drained residue) is contacted with a dilute solution of a water-soluble inorganic base. Examples of such bases, the concentration thereof in the solution, and additional discussion have been given hereinbefore under the heading The Soluble Inorganic Base.

In using dilute solutions, more particularly dilute aqueous solutions, of a water-soluble inorganic base as an extractant of the organomercaptan-reacted lignin, the extraction temperature may range, for instance, from ambient temperature (about 2030 C.) to about 200 C., more particularly from about 50 or 100 C. to about 180 C. The extraction time may range from about A to about 4 or 5 hours or longer, as desired or as conditions may require. Thus, the extraction temperature and time may be 150170 C. for from 1 to 2 hours, more particularly when the extractant is a dilute aqueous solution of sodium hydroxide containing, for example, from about 0.5 to about 5 weight percent of NaOH, and preferably about 1 to 2 weight percent.

Although water alone is the preferred liquid medium in which the water-soluble inorganic base is dissolved, it is not essential to use water as the solvent for the base. For example, one may use in lieu of all or any part of the water an organic solvent in which the inorganic base is soluble, e.g., a lower alkanol such as methanol, ethanol, and the normal and isomeric forms of propyl through amyl alcohols. When the extraction is carried out at temperatures above the boiling point of the solvent or solvent mixture employed, it is effected under pressure and/or reflux conditions as may be required or desired for economical or other reasons. Depending upon the particular type of extraction vessel employed, the residue undergoing extraction may or may not be agitated during the extraction process.

At the end of the extraction period the crude pulp is separated from the extraction liquor, e.g., by filtration, after which it is washed throughly with a liquid washing medium, more particularly water and, specifically, hot water. After Washing it is mechanically defibrated with water (for instance, in a laboratory with a Sprout-Waldron pre-refiner), after which it is screened to remove the residual liquor and to provide a uniform product.

In recovering the mercaptan-reacted lignin and thereby purifying the liquid extraction agent, the extraction liquor may be treated with, for example, CO or a dilute mineral acid such as HCl; an inorganic salt; or an extractantcompatible liquid in which the lignin is insoluble, e.g., alcohol, in order to precipitate the ligneous material for recovery. Dialysis also may be employed to separate the lignin from the extraction liquor. Or, the liquor containing the dissolved lignin may be passed through an anion-exchange resin in free-base form thereby selectively to adsorb on the resin anionic materials contained in the liquor while the lignin in purified form passes through the resin for subsequent evaporation of the eluate and recovery of the lignin. This latter technique is more fully described and broadly and specifically claimed in the copending application of William H. Greive and Karel F. Sporek, Ser. No. 418,872, filed Dec. 16, 1964, now abandoned and assigned to the same assignee as the present invention.

Bleaching and/or drying steps are optional depending upon the end-use. If bleaching is to be effected, it is usually done before drying the pulp. Because digestion is carried out in a treating liquor whch is at least initially alkaline (and may be alkaline throughout the whole cooking period), in some cases it may be desirable, prior to the bleaching step, to wash the crude pulp with a dilute aqueous solution of an inorganic acid, e.g., a 5% aqueous HCl solution, thereby to insure a more complete and efficient bleaching action than when bleaching is effected in the absence of such a dilute acid wash. The pulp, with or without further treatment as may be required for the particular end-use, is then suitable for utilization in making any desired cellulosic product including, for example, paper and related products, cellulose acetate, cellulose xanthate, regenerated cellulose, ethyl cellulose, etc.

One of the advantages of the instant invention is the flexibility with which it lends itself to the obtainment of highor moderate-yield pulps or alpha-cellulose merely by changing such operating parameters as pH, time and temperature of digestion, and amount of TGA and/or other organomercaptan employed. Furthermore the method makes possible the recovery of a relatively high proportion of the organic material that is left in the treating liquor.

In other words, the operating parameters can be varied to obtain lignin and/or pulp or cellulose residue having the desired properties. The conditions can be varied to produce pulp (hence also paper) having a wide range of physical properties; and, additionally, a recoverable lignin having a wide range of utility. For example, the recovered lignin can be utilized as a thermosetting resin, as a coating material or as a component of coating compositions, as a starting material for making chemical compounds and various chemical compositions, and f r many other purposes.

Surprisingly it was found that the organomercaptan-reacted lignin not only can be readily separated from the wood, thereby permitting easier difibration of the wood chips; but also that the isolated lignin derivatives can be hydrolysed in an alkaline ethylene glycol medium whereby there is obtained a higher percentage of water-soluble material of lower molecular weight than heretofore could be obtained by conventional pulping processes In other words, the method of this invention makes it possible to isolate lignin polymer in a non-condensed state such that it is amenable to controlled hydrolysis to smaller units. Thus, the present invention provides a pulping technique whereby the lignin polymeric material is protected or prevented from reacting with itself during its isolation and under conditions that yield a good grade of pulp.

In order that those skilled in the art may better understand how the present invention can be carried into effect, the following examples are given by way of illustration and not by way of limitation. All parts and percentages are by weight unless otherwise stated.

9 EXAMPLE 1 This example illustrates the digestion of a softwood, specifically isopropanol-extracted southern pine, with an organomercaptan, specifically TGA, under the alkaline conditions employed in practicing this invention; and, in a subsequent step, extracting the mercaptan-reacted lignin still retained by the treated wood with a water-soluble inorganic base, more particularly a dilute aqueous solution of sodium hydroxide. I

The procedure employed to extract the southern pine in chip form involves a vapor extraction of the chips with a reboiler wherein the extractives are accumulated and the solvent, isopropanol, is flashed 01f. Isopropanol (about 1.5 liters) is used to extract 1 kg. of southern pine chips. (The same apparatus and procedures are useful in removing extractives from chips of cypress, aspen, black gum and other lignocellulosic materials that are processed in practicing the present invention.)

The treating liquor was prepared by dissolving 77.2 g. TGA in 1.8 liters of water containing 67.0 g. NaOH. The initial pH of this solution was 11.4. In this run the amount of TGA/100 g. chips was 19.3 since 430 g. (399.9 g. on an O.D. basis) of extracted pine chips were used. In another run the treating liquor was prepared by dissolving 25.7 g. TGA in 1.8 liters of water containing 23.0 g. NaOH. The liquor used in this second run, which had an initial pH of 11.8, also was added to approximately 400 g. (O.D. basis) of isopropanol-extracted southern pine chips. Hence the amount of TGA/ 100 g. chips was 6.4 g. in this run. In both runs the ratio of liquor to O.D. wood was approximately 4.5 to 1. In both runs the chips and treating liquor were cooked (digested) in a one-gallon autoclave.

The charge to the autoclave was brought to a temperature of 150 C. over a period of from 50 to 60 minutes, and in each run was held at that temperature for 3 hours. Heating was discontinued, and the charge was allowed to cool for 50-60 minutes to about 90100 C.

The charge was then removed from the digester and filtered to separate the TGA-reacted chips from the spent liquor. The chips were washed with hot water to remove the residual cooking liquor.

The TGA-reacted lignin in the chips was extracted by placing the chips in a vessel containing about 1500 ml. of a 1% aqueous solution of NaOH, and heating the extractant and chips at a temperature of about 96 C. for 1 hour.

The resulting crude pulp was separated from the extraction liquor by filtration, washed thoroughly with hot water, mechanically defibrated with water in a laboratory Spr-out-Waldron pre-refiner, and screened to remove the residual liquor and provide a uniform product for testing.

The cellulosic pulp obtained as described above was then made into papers for testing purposes in the following manner:

The wet cellulosic pulp, produced as hereinbefore set forth, was dried to 20-30% solids, then made into 8" x 8", 26 lb./MSF basis weight handsheets for testing in the following manner: A minima of three aliquots of the experimental pulp, in an amount based on the ovendried weight of the Wet pulp, where refined with water at 1% consistency for varying periods of time in a Mead Laboratory Refiner (manufactured by The Bauer Bros. Co., Springfield, Ohio). The degree to which each pulp aliquot was refined, as determined by measuring the drainage rate of the pulp in a Slowness Tester (manufactured by Williams Apparatus Co., Watertown, N.Y.), was controlled as much as possible so as to provide three or more refining points bracketing 55 seconds Williams Slowness. Each of the refined pulp slurries was diluted to 0.5% consistency and uniformly mixed prior to mak' ing the handsheets. The handsheets were formed in an 8" x 8" Williams sheet mold from aliquots of the pulp slurry that were measured volumetrically for producing 26 lb./MSF sheets. The pulp consistency on forming the sheets was adjusted to 0.05% by further dilution of the pulp aliquot in the mold. The seven or more sheets (wet Webs) formed from each batch of pulp slurry were couched from the wire of the mold onto standard 12" x 12" TAPPI blotters, then stacked between blotters with six blotters separating the sheets. The stack was then pressed for 5 minutes at 150 psi. gauge pressure on a Williams press (manufactured by Williams Apparatus Company). The pressed sheets, retained on the couch blotters, were dried at 260-280 F. on a steam-heated Noble and Wood drier, with the sheet contacting the drum for approximately 2 minutes. After removing the blotters, the dried sheets were conditioned at 50% relative humidity and 73 F. for a minimum of 24 hours before testmg.

More detailed information on the cooking and extraction conditions and the results obtained are given in Table I-A. The column heading, Percent Lignin in Pulp, i.e. Klason lignin, refers to the percentage obtained when the pulp is tested for acid-insoluble lignin using the apparatus and procedure set forth in TAPPI Standard Test Method T222 m-54. The Williams Slowness test, which measures the rate in seconds per liter at which one liter of pulp at 0.3% consistency drains at 20 C. in the Williams Slowness instrument, was used as previously described in order to measure the degree to which the pulp was refined in the Mead refiner.

The results of tests on papers (handsheets) made from the pulps produced as described in Table IA are given in Table I-B. The values for the paper characteristics listed under the columns headed Density, Caliper, Tensile, Stretch and Tear are the results obtained when the respective handsheets were tested using apparatus and procedures set forth where indicated below:

TAPPI Standard Density T411 m-44 Caliper T411 Tensile T404 os-61 Stretch T457 Tear T414 ts64 The only exception from the above-identified test methods was that 5 sheets, each 49 square inches in area, were used in the Density test.

Tables IA and I-B follow:

TABLE IA.'IGA PULPING OF ISOPROPANOL-EXTRACTED SOUTHERN PINE CHIPS Mead Pulp Percent Ratio Refining Williams Cook TGA/ Yield, Lignin Lignin Time, Slowness, No. g. Wood pH Range percent inPulp CH0 2 see. sec.

. 60 5 1-A 6.4 11.8 (initial) to 7.6 79 26 0.35 11 33 60 5 l-B 19.3 11.9 (initial) to 9.4 70 22 0.28 120 14 91 Percent yield is based on initial eight of the wood.

2 CHO=pulp carbohydrate.

TABLE IB.TESTS ON 26 LBJMSF PAPERS MADE FROM PULPS OF COOK NOS. 1-A AND 1-B Dry Tensile Papers from Density, Caliper, Stretch, Tear,

Pulps 01 p.e. mils percent Lbs/1n. p.s.i. g./16 sh.

ll-A 19. 1 1, 350 87 From a comparison of the data given in Tables IA and I-B, and especially a comparison of the data on the papers made from the pulp of Cook 1-A with that obtained when the papers were made from the pulp of Cook l-B, it will be noted that a better pulp is obtained (as evidenced by the properties of papers made therefrom) when a higher amount of TGA, specifically 19.3 g./l00 g. wood vs. 6.4 g./100 g. wood, is employed. Especially significant are the stretch values at a 70% pulp yield that are shown in Table I-B for the papers made from the pulp of Cook 1-B. These values are comparable to stretch values of similarly made handsheets produced from kraft pulps wherein the pulp yield is usually only about 52-53%. Such results were wholly unpredictable and unobvious.

EXAMPLE 2 This example illustrates the pulping of an isopropanolextracted hardwood, specifically an isopropanol-extracted black gum wood. A treating liquor was prepared as in Example 1 by dissolving about 21.8 g. TGA in 1.8 liters of water containing an amount of NaOH such as would provide a liquor having an initial pH of 11.7. The amount of TGA/ 100 g. O.D. black gum chips was 6.4 g., since 358 g. (340.8 g. on an OD. basis) of the said chips were treated with the liquor. The ratio of liquor to DD. Wood was approximately 4.5 to l. The digestion temperature was 130 C., while the time of digestion was 1 hour. The initial pH of 11.7 of the treating liquor was reduced to a pH of 8.7 at the end of the cooking period. Except as indicated above, the apparatus and procedure were otherwise essentially the same as described in Example 1.

The chips were separated from the cooking liquor by filtration, washed with hot water, and then extracted with 2 liters of an aqueous 1% solution of sodium hydroxide for 1 hour at 96 C., followed by suspending the extracted chips in 3 liters of hot tap water. The chips were separated from the water by filtration, and refined as described in Example 1. The cooked and extracted chips required medium refining, the refining behavior being of the same general character as that of the cooked and extracted aspen chips of Example 3 that follows.

The pulp of this example is especially suitable for use in making corrugating media.

EXAMPLE 3 Same as in Example 2, including the post-extraction with aqueous NaOH, with the exception that a different hardwood, more particularly isopropanol-extracted aspen, was used; and the pH range of the treating liquor was from an initial pH of 11.5 to 8.6. The yield of pulp was 82%. The percent of lignin in the pulp was 16%. The ratio of lignin to pulp carbohydrate was 0.19. The Mead refining time and the Williams SloWness values are tabulated below:

Mead refining time, Williams Slowness,

seconds: seconds 30 11 60 30 90 51 The physical properties of handsheets made from the refined pulp in the manner described in Example I are shown in the following Table II.

TABLE II.-PROPERTIES OF PAPERS MADE FROM THE PULP OF EXAMPLE 3 same general procedure desired above but under slightly different conditions, there was obtained approximately a 75% yield of pulp containing about 20% lignin and wherein the ratio of ligin to carbohydrate was 0.25. Handsheets made from this pulp had a density of about 30 p.c.f., a dry tensile strength of about 4000 p.s.i., a tear value of about g./l6 sheets, a Mullen value of about 84 p.s.i., and a ring crush value of about 43 pounds. (The Mullen and ring crush values were determined in this example and in other examples that follow using the apparatus and procedures of TAPPI Standards T403 ts-63 and T472 m-Sl, respectively.) As compared with handsheets made by the kraft process, the aforementioned values for tensile and ring crush are superior at equivalent tear values. Also, the yield of 75% is much higher than the average yield of about 52-53% for papers made by the kraft process.

EXAMPLE 4 This example illustrates that it is not necessary to use separately prepared and/or further refined organomercaptan, specifically TGA or a salt thereof, in carrying the present invention into effect. For example, crude TGA can be used.

Crude TGA was prepared by combining 51.3 g. chloroacetic acid, 21.7 g. sodium hydroxide, and 51.9 g. of a 73.5% aqueous solution of sodium sulfhydrate (NaSH) in a total water solution of 600 ml. The resulting solution was heated with stirring for 1 hour at a temperature of from 50 C. (initial temperature) to C. This solution (containing an excess of NaSH over the stoichiometrical amount required for the formation of thioglycolic acid from chloroacetic acid) was used directl as a treating liquor in the digestion of isopropanol-extracted southern pine chips following the same general procedure as was used in Example 1.

The amount of crude TGA employed (on a solids basis) was 19.3 g./ g. (O.D. basis) of isopropanol-extracted pine chips. The pH range of the treating liquor was from an initial pH of 8.7 to a final pH of 6.1. The temperature of extraction with 1% aqueous NaOH (as in Example 1) was 98 C., and the time of extraction was 1 hour.

The yield of pulp was 78.4%, and the lignin content thereof was 22.0%. It contained about 0.4% sulfur and 8.48% pentosans.

The technique of this example has important economical advantages from a raw materials standpoint (refined or performed TGA is more expensive than chloroacetic acid and NaSH). It also has advantages in providing a pulp in a higher yield with no lessening of quality and, in many cases, even better quality in certain respects as compared with the use of TGA. Note the aforementioned yield of 78.4% pulp with a lignin content of 22% as compared with the 70% yield and 22% lignin content of Example 1-B (Table I-A) wherein 19.3 g. of refined TGA was employed.

13 EXAMPLE This example illustrates two-stage pulping with TGA of isopropanol-extracted southern (specifically Valdosta) pine and wherein the extraction conditions were varied. The general procedure was essentially the same as in Ex ample 1. In four of the five runs that were made the amount of TGA in the treating liquor was g./100 g. O.D. wood. In a fifth run the amount of TGA was 21.2 g./100 g. O.D. wood, which Was equivalent to 12.8 g TGA +5.1 g. NaSH/ 100 g. O.D. wood. In all runs the treating liquor had an initial pH of 11.0, and a pH at the end of the individual runs ranging from 5.5 to 9.4. In all runs the treating or digestion time was 2 hours at the maximum temperature of 170 C. The pulp yields ranged from 60% to more than 72%.

Other details of the digestion and extraction conditions, pulp yield, percent lignin in pulp, carbohydrate yield and refining data are given in Table III-A. The results of tests on handsheets made from the pulps of the cooks of Table IIIA are given in Table III-B. The values for brightness in Table IIIB (also in Tables IVB and VB that follow) are determined using the apparatus and promixtures of, by weight, a major proportion (-85%) of a soft wood, specifically southern pine, and a minor proportion 1530%) of black gum wood.

The general procedure was essentially the same as in Example 1. In all four of the runs that were made the amount of TGA in the treating liquor was 10 g./100 g. O.D. wood. In all runs the treating liquor had an initial pH of 11.0, and a pH at the end of the individual runs ranging from 5.2 to 5.5. The digestion temperature (maximum) was 170 C., and the digestion time at this temperature was 1 hour in each of two runs and 2 hours in the other two runs. The post-extraction time was 1 hour in all runs, using an aqueous solution of either 1 or 2% NaOH, and an extraction temperature of either 100 or 150 C. The pulp yields varied from about 59% to about 65%. Other details of the cooking, post-extraction (i.e., second-stage treatment) and refining conditions, as well as pulp and carbohydrate yields and percent lignin in the pulp are given in Table IVA. The results of tests on handsheets made from the pulps of the cooks of Table IVA are given in Table IVB.

Tables IVA and IVB follow.

TABLE IVA.-TWO-STAGE TGA PULPING OF BLACK GUM AND MIXTURES OF PINE AND BLACK GUM Mead Percent, Percent, Percent, Ref. Williams Den- Cook pH Time, Pulp Pulp CHO Time, Slowness, sity,

No. Type Chem./100 g. Wood Range hrs. Extraction Conditions Yield Lignin Yield sec. sec. p.c.t'.

411-2. 100 13.61.... 11. o-5.2 1 17 NaOH; 1 111.; 100 0 59.4 21.4 46.7 so 27.0 29.3

4al MOE} o 107 43. s 31. 4

10.0 g. T GA 0 4b- 100 B.G. 11. 0-5. 2 1 27 NaOH, 1 hr., 100 C 61. 8 21. 6 48. 5 27. 5 28. 2

% p1ne+ 10.0 g. TGA' 7.2 0

40-2... 11. 05. 5 2 17 NaOH, 1 hr. 250 C 64.8 26.4 47. 7 46.1 31. 4

' 70% p1ne+ 10.0 g. TGA; 7.2 o

4d2 11. 0-5. 4 2 17 NaOH, 1 hr., 150 C 62. 3 25. 5 46. 4 70 22. 9 28.0

4d-3 30% NaOH- 0 100 55. 7 s3. 4

1 B.G.=black gum.

cedure set forth in TAPPI Standard T452 m-58.

Tables III-A and III-B follow.

TABLE IIIA.TWO-STAGE TGA PULPING OF ISOPROPANOL-EXTRAOTED SOUTHERN PINE Mead Percent, Percent, Percent, Ref. Williams Cook pH Pulp Pulp H0 Time, Slowness, N0. Chem./100 g. Wood Range Extraction Conditions Yield Lignin Yield see. sec.

3a-1 50 10. 7 311-2 10.0 g. TGA; 7.1 g. NaOH 11. 0-6. 0 2% NaOH; 1 111.; 100 C 72. 3 25. 2 54.1 100 18. 4 321-3 135 67.0 3b-1. 50 8. 7 3b2 10.0 g. TGA; 7.0 g. NaOH 11. 0-5. 5 1% NaOH; 1 hr.; 150 0-.-.-- 67. 0 26. 7 49.1 100 34. 8 3b-3 84. 8 30-1 40 8. 3 3c-2- 10.0 g. TGA; 7.1 g. NaOH 11. 05.7 1% NaOH; 1 hr.; 150 C 65. 6 26. 2 48.4 3c- 3d-1 60 9. 1 3d-2 10.0 g. TGA; 7.1 g. NaOH 11.0-6. 0 2% NaOH; 1 hr.; C 60. 0 20. 2 47. 9 100 35.3 3d-3 110 48. 6 3e-1 80 7. 7 3e 21.2 g. TGA; 15.5 g. NaOH 11. 09.4 2% NaOH; 1 hr.; 100 C 67.3 20.2 53.7 133.8 3e- 1 CH0 =carbohydrate in pulp.

TABLE IIIB.TESTS ON 26 LBJMSF PAPERS MADE FROM PULPS OF COOKS OF TABLE III-A 2 Equivalent to 12.8 g. TGA+5.1 g. NaSH.

TABLE IVB.TESTS ON 26 LBJMSF PAPERS MADE FROM PULPS OF COOKS OF TABLE IVA P apers Pacers from Dry Ring Bright- Irozn Dry Ring Bright- Pulps of Density, Tensile, Tear, Mullen, Crush, ness, Pulps of Density, Tensile, Tear, Mullen, Crush, nes Cook N o. p.c.f. p.s.l. g./16 sh. p.s.1. lbs. percent Cook No. p.c.f. p.s.i. g./16 sh. p.s.i. lbs. percent 60 26. 0 2, 556 105 43 57 23. 0 27. 0 2, 491 196 59 48 19. 5 29. 3 3, 285 127 55 61 23. 8 28.7 3, 701 183 62 51 19. 5 31. 4 4, 192 132 62 63 23. 5 32. 6 4, 063 164 76 65 19. 5 25.4 2, 369 107 45 40 22.8 24. 7 2, 296 253 48 46 18. 2 28. 2 3, 401 129 52 53 22. 3 29. 9 3, 576 189 70 61 18. 4 30. 2 3, 773 139 62 66 23. 0 32.8 4, 445 77 63 18. 2 65 25. 9 2, 668 229 53 49 16. 6 24. 4 1, 627 230 46 43 17 5 31. 4 3, 985 183 77 59 17. 9 31. 7 724 190 72 59 18. 5 32. 4 4, 095 71 58 18. 5 32. 9 4, 430 79 56 18. 5 25. 5 2, 241 256 47 47 16. 9 28.3 3, 163 255 63 48 17.4 28. 0 3, 189 209 64 56 17.4 31. 8 4, 140 214 80 62 17.9 33.4 4, 342 174 77 60 17. 0 32. 9 4, 684 194 83 65 18. 3 .1 3 3 8 52 3'8 70 32. 3 8 l 1 2 35.1 5,118 155 94 57 21. 0 EXAMPLE 7 This exam le illustrates a simulated laborator kraft EXAMPLE 6 P Y This example illustrates the two-stage pulping with TGA of a hardwood, specifically black gum wood, and

cook of isopropanol-extracted southern pine, more particularly Valdosta pine. The cook was made primarily for the purpose of comparing the properties of the pulp 15 and of handsheets made therefrom with the corresponding products of Examples and 6. Details of the digestion conditions, as well as with regard to yields and refining conditions, are given in Table VA. The results of tests on papers (handsheets) made from the pulp identified in Table VA are given in Table VB.

FORMULATION FOR COOK NO. 5-a

The above liquor was prepared by dilution of concentrated mill white liquor. The active alkali concentration of 47.1 g.p.l. as 'Na O is the same as that of a mill cook at 16% active alkali on the wood and 3.4 liquor-to-wood ratio. As a result of the high liquor-to-wood ratio required in this cook (9:1), the active alkali on the wood was at a high level (42.4%) as compared to mill pulping.

Tables VA and VB follow.

TABLE VA.-KRAFT COOK OF ISOPR still obtain pulps having almost the properties of kraft pulp. This is not true with respect to the kraft process of pulping wherein only about 10-15% of hardwood admixed with softwood can be tolerated. From this it will be seen that the process of this invention is eminently suitable for use in plants located in areas where both hardwoods and softwoods are available for pulping and wherein the economic advantages of a single design of plant adapted to handle both types of woods are desired. From the foregoing general description of the instant invention it will be seen that it is materially and unobviously different from that described by Holmberg in the publication cited in the early part of this specification. It is also separately and patentably distinct from the invention disclosed and claimed in my copending application Ser. No. 606,024, filed concurrently herewith, and which is concerned with the use of a combination of an organomercaptan and a hydrotrope agent in a method for pulping lignocellulosic material. Likewise the present invention is separately and patentably distinct from that disclosed and claimed in the copending applications, also filed concurrently herewith, of William E. Fisher and Shibley A. Hider, Ser. No. 605,978; Ser. No. 606,025; and Ser. No. 605,957. All of the foregoing applications are assigned to the same assignee as the present invention.

I claim: 1. The method of pulping lignocellulosic material by a two-stage treatment which includes the steps of (A) digesting said material with a treating liquor containing an agent reactive with the said lignocellu- OPANOL-EXTRACTED SOUTHERN PINE Wil Mead liams Percrnt Percent Percent Ref. Slow- Cook pH Temp., Time, Pulp Pu lp 0110 Time, ness, No. ChemJ/IOO g. Wood Range 0. Hrs. Yield Ligmn Yield sec. see 5a-1. 6.1 g. Nags; 14.4 g. NaOH; 120 9. 7 5a-2 4.8 g. Na CO (16% 13. 4 150 2 55. 8 17. 6 46.0 150 24. 9 5a-3- AA?) 180 56. 7

1 Mill Kraft White Liquor. 2 A.A.=active alkali as N320 TABLE VB.-TESTS ON 26 LB. [MSF PAPERS MADE FROM PULPS OF COOK OF TABLE VA Papers from Pulps 0! Density, Dry Tensile, Tear, Mullen, Ring Brightness, Cook No. p.c.t. p.s.i. g./16 sh. p.s.1. Crush, lbs. percent 28. 5 2, 930 257 76 46 17. 7 31. 6 3, 535 228 86 57 17. 7 34. 7 4, 640 195 89 63 17. 5

A comparison of the data in the tables of Examples 5, 6 and 7 shows that an organomercaptan, specifically TGA, can be used as a pulping chemical in making pulps from hardwoods, softwoods and mixtures thereof. The yields of pulps of 60 to 72% (Table III-A of Example 5) are substantially higher than the 55.8% yield of kraftprocess pulp (Table VA of Example 7).

Most properties of the pulps (and of papers made therefrom) of Example 5 are comparable to the kraft pulp (and of papers made therefrom) of Example 7, but the papers of Example 5 have appreciably better color as shown by their much higher brightness values as compared with those of the kraft papers.

The data in the tables of Example 6 show that hardwood can be pulped by the process of this invention to obtain a pulp in a yield of from about 59-65% from which are obtainable papers much better in color and with other properties almost as good as papers made from a kraft pulp that was obtained in a yield of 55.8%. Note the higher brightness values of 23% and better for the papers of Table IV-B as compared with the brightness value of about 17.5 (Table VB) for the kraft paper of Example 7.

Certain of the data in the tables of Example 6 also show that at least hardwood can be mixed with 70% softwood to obtain good-quality pulps; and the available evidence indicates that this could be increased up to -50 mixtures of hardwood and softwood and losic material which is comprised of an organomercaptan in an amount corresponding to at least 5 weight percent based on the weight of the oven-dried lignocellulosic material, said treating liquor having an initial pH within the range of from 7.6 to about 12.0, the temperature and time of digestion being sufficient to convert the lignocellulosic material to 9. treated material containing mercaptan-reacted lignin; and

(B) extracting mercaptan-reacted lignin retained by the digested lignocellulosic material by contacting it with a dilute solution of a water-soluble inorganic base.

2. The method as in claim 1 wherein the lignocellulosic material is hardwood.

3. The method as in claim 1 wherein the lignocellulosic material is softwood.

4. The method as in claim 1 which includes the step of removing the excess liquor from the treated material from step A; and the mercaptan-reacted lignin retained by the digested lignocellulosic material, and from which the excess liquor has been removed, is extracted with a dilute aqueous solution of an alkali-metal hydroxide or carbonate.

5. The method as in claim 4 which includes the step of washing the residue that remains after removing the excess liquor from the organomercaptan-treated lignocellulosic material; and the mercaptan-reacted lignin in 1 7 the washed residue is extracted with a dilute aqueous solution of sodium hydroxide.

6. The method of pulping lignocellulosic material by a two-stage treatment which includes the steps of (A) digesting said material with a treating liquor containing an agent reactive with the said lignocellulosic material which is comprised of an organomercaptan in an amount corresponding to from about to about 50 weight percent based on the weight of the oven-dried lignocellulosic material,

said organomercaptan being one represented by the general formula wherein Z represents an alkylene radical containing from 1 through 10 carbon atoms; R represents a member of the group consisting of (a) hydrogen, b) alkyl radicals containing not more than 10 carbon atoms, and (c) a salt-forming cation; and n represents an integer from 1 up to that of the combining power of the alkylene radical represented by Z, said treating liquor having an initial pH within the range of from 7.6 to about 12.0, and the temperature and time of digestion being sufficient to convert the lignocellulosic material to a treated material containing mercaptan-reacted lignin; (B) removing the excess liquor from the treated material from step A; and (C) extracting mercaptan-reacted lignin retained by the digested lignocellulosic material, and from which the excess liquor has been removed, by contacting it with a dilute aqueous solution of sodium hydroxide. 7. The method as in claim 6 wherein the organomercaptan is one represented by the general formula wherein n represents an integer from 1 to 8, inclusive, and R represents a member of the group consisting of (a) hydrogen, (b) alkyl radicals containing not more than 8 carbon atoms, and (c) a salt-forming cation.

8. The method as in claim 7 wherein the organomercaptan is thioglycolic acid.

9. The method as in claim 6 wherein the organomercaptan comprises thioglycolic acid and the dilute aqueous solution of sodium hydroxide used in step C is one that contains from 0.5 to 5 weight percent of NaOH.

10. The method as in claim 6 wherein the organomercaptan comprises thioglycolic acid; the treating liquor has an initial pH within the range of from 7.6 to about 12; the digestion temperature is within the range of from C. to 200 C.; and the dilute aqueous solution of sodium hydroxide is one that contains from 1 to 2 weight percent of NaOH.

11. The method as in claim 10 wherein the organomercaptan comprising thioglycolic acid is present in the treating liquor in an amount corresponding to from about 10 to about 30 weight percent based on the weight of the oven-dried lignocellulosic material; the treating liquor has an initial pH within the range of from about 8.0 to about 11.0; the digestion temperature is within the range of from about C. to about 190 C.; and the digestion time within the aforesaid temperature range'is from about /2 to about 3 hours.

12. The method as in claim 11 wherein the digestion temperature is from C. to C.; and the digestion time at the maximum temperature of digestion is from 1 to 3 hours.

13. The method as in claim 11 which includes the step of water-washing the residue that remains after removing the excess liquor from the lignocellulosic material that has been digested in a treating liquor containing an organomercaptan comprising thioglycolic acid.

References Cited Wood Chemistry, Wise and John, 2nd ed., vol. I, published by Reinhold Pub. Corp., New York, N.Y., 1952, p. 435 and p. 498.

HOWARD R. CAINE, Primary Examiner U.S. Cl. X.R. 162-7]. 77 

